Electrically functional three-dimensional structures have a variety of uses in biomedical, chemical, physical, and electronic applications. Non-limiting examples of these applications include electrically active microneedle arrays for transdermal gene therapy or drug delivery by electroporation, high-aspect-ratio metal pillar arrays for solid fuel combustion, and electromagnetically radiating antenna arrays, among other uses.
In many of these applications, miniaturization creates the need for high-aspect-ratio (i.e., height divided by width) conductive structures. For example, in microneedle applications high-aspect-ratio structures are preferred for their rigidity and consequent ability to penetrate human skin in vivo. In electrical applications, circuit densities continue to increase. As the circuits become smaller, the widths of vias, contacts, and other features, as well as the dielectric materials between them all decrease significantly, whereas the thickness of the dielectric layers remains substantially constant. The result being that many features take on increasingly larger aspect ratios. While conventional structures have been created through well-known techniques such as micromachining of solid conductors, these approaches are not cost-effective and therefore cannot be scaled to larger production.
Electrodeposition has been proposed as an economically viable alternative for making conductive three dimensional structures. Metal electrodeposition is generally well-known and can be achieved by a variety of techniques. A typical method generally comprises depositing a barrier layer over the feature surfaces, depositing a conductive metal seed layer over the barrier layer, and then electroplating a conductive metal over the seed layer to cover the structure. An example of this technique is described in U.S. Pat. No. 7,196,666 by Allen et al., entitled “Surface Micromachined Millimeter-Scale RF System and Method,” the disclosure of which is incorporated herein by reference. Although electrodeposition may provide cost savings over micromachining of solid conductive structures, an additional challenge exists in patterning high-aspect-ratio three-dimensional structures due to difficulty in non-planar surface patterning using conventional lithographic approaches. It therefore would be desirable to provide improved methods of efficiently fabricating electrically-conductive structures, particularly high-aspect-ratio structures, in a process that can be scaled for mass-production.